The present invention relates to a method for illuminating an object to be imaged by a microscope, in particular, a surgical microscope, where the object is illuminated by primary light of a first spectral intensity distribution, as well as by secondary light of a second spectral intensity distribution, the secondary light arising from the scattering of the primary light. The present invention also relates to a surgical microscope having an illuminating device for producing primary light for illuminating an object to be imaged.
Without limiting universality, the following explanations regarding the present invention relate to the special case of what is generally referred to as “red reflex” illumination in the context of surgical microscopes for ophthalmology, where light reflected from the retina is used for surgical purposes. Since the light reflected at the retina has intensity maxima in the red spectral region, one speaks of what is generally referred to as “red reflex.” In this application, the object to be examined (the anterior chamber of the eye from the lens to the cornea) is illuminated by primary light of a first spectral light intensity distribution. Light reflected from the retina of the eye is backscattered or reflected towards the object and thus illuminates the object from below, quasi as background or retroillumination. When considered strictly in physical terms, the primary light is not reflected at the retina, but is backscattered there with a solid angle-dependent intensity profile. For the sake of simplicity, the terms “scattering” and “reflection” are to be treated synonymously in the context of the present application, particularly for the application case discussed here.
In cataract surgery, in particular, where the natural lens of the human eye is replaced by an artificial lens, the “red reflex” is utilized to readily detect, under the red retroillumination, any material (bits of tissue) remaining following removal of the natural lens, and to easily remove the same. The “red reflex” is all the more pronounced, the smaller the illumination angles are, the assumption being that the illumination beam path extends through the main objective of the surgical microscope, and that the axis of the main objective is to be considered as the reference axis. Illumination angles in the range of between −2° and +2° promise a is good “red reflex” illumination.
In addition to the application cases described here, the present invention is also generally applicable to the field of microscopy, to the extent that an object is illuminated by primary and secondary light, the secondary light arising from the scattering or reflection of the primary light.
Because the illumination angles used in ophthalmology in the context of the “red reflex” illumination are small and, due to the latest developments, becoming increasingly smaller (to the point of a true 0° illumination), it is becoming increasingly more likely that a patient's eye movement will cause his/her macula to be irradiated directly and dangerously for an extended period of time. The macula (also referred to as “yellow spot”) is the region of the human retina having the greatest density of visual cells, and it contains the site of the highest acuity vision. It is imperative to prevent direct irradiation of the macula and any significant endangerment to the eye of the patient resulting therefrom. It is self-evident that a prolonged irradiation of the retina should also be altogether avoided, particularly in view of thermal and phototoxic effects.
In this field, the surgical microscope OPMI Lumera T from Zeiss is known from the related art. The corresponding 2007 prospectus of Carl Zeiss Surgical GmbH, page 4, discusses the “red reflex,” which has a bright and stable appearance when the microscope is positioned over the eye of the patient and the illuminating device is switched on. In fact, however, a yellow “red reflex” is discernible in the corresponding image, possibly suggesting that the macula is illuminated.
The technical specifications pertaining to the mentioned product of Carl Zeiss Surgical GmbH are described in the U.S. Patent Application 2004/0227989 A1. It discusses a surgical microscope for ophthalmology where the illumination is directed through the main objective of the microscope. The illuminating device is essentially composed of an optical fiber (fiber illumination), a downstream collimator, as well as of a reflecting mirror, which directs the light that is bundled and collimated by the collimator through the main objective of the microscope into the object plane. The object plane extends through the anterior portion of the eye. A second reflecting mirror configured closer to the observation channels of the stereo microscope directs a portion of the illumination light at an angle of between −2° and +2° relative to the main observation beam paths through the main objective to the object plane, this light being used to produce the “red reflex.” This illumination beam is scattered at the retina and reflected back with a solid angle-dependent profile, so that a red retroillumination of the object plane is obtained. Since the mentioned design does not allow the “red reflex” to be maintained at a sufficient intensity level, particularly in the case of movement of the eye, the cited document provides for an illumination ring composed of LEDs (light-emitting diodes) to be placed around the observation channels defined by the zoom systems of the surgical microscope. Each LED emits light in the red spectral region that is nearly completely reflected or scattered back by the retina, so that the eye is protected from thermal stress. In another specific embodiment, a conventional illuminating device is used where an LCD (liquid crystal display) array is configured downstream of the collimator and a white light source. In a central region, the LCD array transmits only red light, while, in an outer annular region, it transmits white light. An inner red illumination cone is thereby formed that is surrounded by an outer white illumination cone. Thus, in accordance with this document, the protective measures are limited to an irradiation of the retina using a suitable (red) spectral region in order to protect the same from thermal damage.
U.S. Pat. No. 6,914,721 B2 of the Applicant, which relates to the same field, describes using a prism combination to deflect the illumination beam path into two different regions of the main objective of the microscope, in order to obtain an oblique and a 0° illumination of the eye to be examined. The oblique illumination is used in this case for the actual object illumination, while the 0° illumination is used for producing the “red reflex.” As a protective measure, a shifting mechanism is provided which removes the prism combination from the optical axis of the main objective in a direction perpendicularly to this optical axis. In the shifted state, two oblique illumination beams are generated so that no “red reflex” can occur. The prism combination is moved away from the centered position (with “red reflex”) into the shifted position (without “red reflex”) as a function of the operating distance and/or as a function of the luminous intensity, in order to protect the retina of the patient's eye.
Consequently, in accordance with the teaching of this document, if a predefined operating distance is fallen short of, or if a predefined illuminating light intensity is exceeded, then, as a protective measure, the “red reflex” is completely prevented by shifting the prism combination.
The U.S. Pat. No. 4,715,704 likewise relates to an ophthalmoscopic surgical microscope, where, to protect the retina of the eye from too high of a radiation load, a retina-protection field stop is provided which is introducible into the illuminating device at a location that is conjugate to the object plane, which, in turn, resides in the anterior portion of the eye (for example, in the cornea). By introducing the field stop for protecting the retina, a central vignetting is achieved, the diameter of the unlighted portion corresponding to the diameter of the pupil of the eye. This protects the retina from further irradiation.
None of the documents mentioned discusses the problem of preventing illumination of the macula during an illumination of the retina.
In another context, the German Patent Application DE 103 41 521 A1 describes a method for determining an object illumination that is adapted to an object under observation, namely for the purpose of enhancing the contrast between healthy and malignant tissue during a microscopic examination. The assumption here is that malignant tissue exhibits a different scattering behavior in the visible spectral region than does healthy tissue. In accordance with the teaching of this document, that wavelength region is ascertained by spectral analysis in which the scattering behavior differences are most visible. The subsequent object examination then takes place using the illumination in the ascertained wavelength region.
Similarly, the German Patent Application DE 103 41 285 A1 describes a surgical microscope which has a spectrometer system that is supplied with scattered illumination light from an operative area in order to spectrally analyze this illumination light. The type of tissue (healthy or malignant) present in the examined operative area can be inferred from the spectral composition of the scattered illumination light. To this end, the operative area is, so to speak, scanned by tiltable deflector elements.